From 9ff77120a59445df617ed15186e46c647912742c Mon Sep 17 00:00:00 2001
From: Alejandro Borbolla <alejandro.borbolla@datadoghq.com>
Date: Tue, 2 Sep 2025 17:05:41 +0200
Subject: [PATCH 1/2] fix(tool): Do not return empty `outputSchema`

If the output schema is not specify, we do not return it as we break the MCP specification that are expecting the following format:

```json
  "outputSchema": {
    "type": "object",
    "properties": {},
    "required": []
  }
```
---
 mcp/tools.go      |  2 +-
 mcp/tools_test.go | 60 +++++++++++++++++++++++++++++++++--------------
 2 files changed, 43 insertions(+), 19 deletions(-)

diff --git a/mcp/tools.go b/mcp/tools.go
index 185aefa69..493e8c778 100644
--- a/mcp/tools.go
+++ b/mcp/tools.go
@@ -607,7 +607,7 @@ func (t Tool) MarshalJSON() ([]byte, error) {
 			return nil, fmt.Errorf("tool %s has both OutputSchema and RawOutputSchema set: %w", t.Name, errToolSchemaConflict)
 		}
 		m["outputSchema"] = t.RawOutputSchema
-	} else {
+	} else if t.OutputSchema.Type != "" { // If no output schema is specified, do not return anything
 		m["outputSchema"] = t.OutputSchema
 	}
 
diff --git a/mcp/tools_test.go b/mcp/tools_test.go
index 270bf64b0..c8deacfea 100644
--- a/mcp/tools_test.go
+++ b/mcp/tools_test.go
@@ -586,27 +586,51 @@ func TestToolWithOutputSchema(t *testing.T) {
 		Email string `json:"email,omitempty" jsonschema_description:"Email address"`
 	}
 
-	tool := NewTool("test_tool",
-		WithDescription("Test tool with output schema"),
-		WithOutputSchema[TestOutput](),
-		WithString("input", Required()),
-	)
-
-	// Check that RawOutputSchema was set
-	assert.NotNil(t, tool.OutputSchema)
+	tests := []struct {
+		name                 string
+		tool                 Tool
+		expectedOutputSchema bool
+	}{
+		{
+			name: "default behavior",
+			tool: NewTool("test_tool",
+				WithDescription("Test tool with output schema"),
+				WithOutputSchema[TestOutput](),
+				WithString("input", Required()),
+			),
+			expectedOutputSchema: true,
+		},
+		{
+			name: "no output schema is set",
+			tool: NewTool("test_tool",
+				WithDescription("Test tool with output schema"),
+				WithString("input", Required()),
+			),
+			expectedOutputSchema: false,
+		},
+	}
 
-	// Marshal and verify structure
-	data, err := json.Marshal(tool)
-	assert.NoError(t, err)
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Marshal and verify structure
+			data, err := json.Marshal(tt.tool)
+			assert.NoError(t, err)
 
-	var toolData map[string]any
-	err = json.Unmarshal(data, &toolData)
-	assert.NoError(t, err)
+			var toolData map[string]any
+			err = json.Unmarshal(data, &toolData)
+			assert.NoError(t, err)
 
-	// Verify outputSchema exists
-	outputSchema, exists := toolData["outputSchema"]
-	assert.True(t, exists)
-	assert.NotNil(t, outputSchema)
+			// Verify outputSchema exists
+			outputSchema, exists := toolData["outputSchema"]
+			if tt.expectedOutputSchema {
+				assert.True(t, exists)
+				assert.NotNil(t, outputSchema)
+			} else {
+				assert.False(t, exists)
+				assert.Nil(t, outputSchema)
+			}
+		})
+	}
 }
 
 // TestNewToolResultStructured tests that the NewToolResultStructured function

From 4fee9866541000918c16af16c6c7a1a0946635b3 Mon Sep 17 00:00:00 2001
From: Alejandro Borbolla <alejandro.borbolla@datadoghq.com>
Date: Tue, 2 Sep 2025 17:17:58 +0200
Subject: [PATCH 2/2] fix(tool): typo in test

---
 mcp/tools_test.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mcp/tools_test.go b/mcp/tools_test.go
index c8deacfea..4bb07aa5d 100644
--- a/mcp/tools_test.go
+++ b/mcp/tools_test.go
@@ -603,7 +603,7 @@ func TestToolWithOutputSchema(t *testing.T) {
 		{
 			name: "no output schema is set",
 			tool: NewTool("test_tool",
-				WithDescription("Test tool with output schema"),
+				WithDescription("Test tool with no output schema"),
 				WithString("input", Required()),
 			),
 			expectedOutputSchema: false,